The use of reactive or excited gases in the treatment of particulate has held much promise for product development. For example, by coating particulate, or changing their surface chemistry, powders which would otherwise be incompatible with a variety of host materials may be rendered compatible therewith. Plasmas could be used to deposit hydrophobic coatings on moisture-sensitive powders to reduce degradation and increase storage time or, conversely, particles can be modified such that they disperse into liquids more readily. However, the promise of this technology has gone largely unfulfilled due to the rigors of the various methodologies employed to date.
McCoy, U.S. Pat. No. 3,305,466 relates to a method and apparatus for reacting gas and solids. The method of McCoy is applicable to a reaction between a gaseous or vaporized reactant and a solid which could be comminuted therewith and which, in comminuted form, is susceptible to the effect of an alternating electric current. According to the method of McCoy, solids which are susceptible to the effect of an alternating electrostatic field are generally heated and introduced into a reaction chamber where they are agitated by the application of an alternating electric field and by the countercurrent flow of a reaction gas. For example, ferrite ore containing tungsten was heated to 300.degree. C. and reacted with carbon tetrachloride. This method is, however, limited. First, the requirement of alternating electric current susceptibility and the application of high temperatures significantly limits the type of particle which may be used and thus treated. Furthermore, this process is not energy efficient. Moreover, as McCoy requires agitation of its particles by the flowing gas, it is limited to operation at relatively high gas pressures. Finally, the reaction gas utilized often presents a safety and a disposal problem.
Treatments using plasma activated species may eliminate many of the disadvantages associated with ordinary gas treatment processes and provide results that are otherwise unobtainable.
Furthermore, plasma activated species treated powders, may advantageously be used in a number of environments. For example, ultra-high molecular weight polyethylene may be used as an additive for thermoplastic composites. Treated rubber may be used in non-slip epoxy flooring. Treated talc, clay, silica, carbon black and ground tires may be used as filler material. Treated pigments may be used for paints and coatings, treated micronized waxes may be used in inks, lubricants and coatings, and treated polymer dispersions may be used in coatings and emulsions. Unfortunately traditional plasma techniques for treating powders or particles are generally batch or modified batch procedures which are not economically feasible. See, for example, U.S. Pat. Nos. 4,423,303, 4,619,861, 4,685,419, 4,810,524 and 4,867,573. In addition, the batch plasma treatment of certain particles is not possible due to their size, shape or density.
Furthermore, particles cannot be treated uniformly in a batch mode because particles in the bulk receive less plasma exposure than particles at the surface, even though new particles are continuously moving to the surface. To mitigate this problem, long treatment times and/or violent agitation are generally necessary. However these treatments preclude the use of delicate or treatment-time sensitive powders and often result in the over exposure of a significant percentage of the treated particles. In the case of, for example, polymer particles, this may lead to discoloration, nonuniformity of structure or properties, and the like.
The present invention makes feasible the use of plasma activated species to treat powders or particulate continuously, and eliminates many of the deficiencies in conventional batch treatments. Specifically, the present invention provides methods and apparatus for treating particulate in a continuous fashion with a plasma activated species. The present invention is particularly well suited for the treatment of particulate which because of size, delicacy, or composition, have not readily lent themselves to such treatments in the past. For example, the present invention is particularly useful for the treatment of polymer particles.